Conditions required for expression of hormone (glucagon or epinephrine) action on adenylate cyclase systems have been examined with cells in culture. Primary rat adult hepatocytes fail to grow in culture but maintain hormone responsiveness provided that insulin and glucocorticoids are present during culture. By contrast, cells (fetal hepatocytes, HeLa, erythroleukemic) that grow in culture media display poor responsiveness to hormones unless differentiation-inducing agents (butyric acid, dimethylsulfoxide, 1-methyl-3-isobutylxanthine) are present. The actions of these agents do not involve effects on the cell cycle.